Bearing steel

ABSTRACT

The present invention relates to a bearing steel comprising at least 0.6 percent by weight of carbon, 0.007 percent by weight or less of phosphorous, and optionally other alloying element(s); the balance being iron, or iron and impurities. The present invention also relates to a bearing consisting of the bearing steel.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a bearing steel. The present inventionfurther relates to bearings made from bearing steel.

BACKGROUND

Bearings, such as roller bearings and ball bearings, are subjected tohigh loads during use. Thus bearings should have high fatigue strengthand in addition high hardness.

Steels with high amounts of carbon are known to have high hardness, butit is a problem that such steels have low fatigue strength.

SUMMARY OF THE INVENTION

One purpose of the present invention is to provide bearing steel withimproved fatigue strength. It is also a purpose of the present inventionto provide bearings produced from the bearing steel according to thepresent invention.

According to a first aspect of the present invention the purpose isaccomplished by a bearing steel comprising at least 0.6 percent byweight of carbon, 0.007 percent by weight or less of phosphorous, andoptionally other alloying element(s), the balance being iron.

It is understood that the balance may, in addition to iron, consist ofimpurities. Such impurities may be impurities and trace elementsnormally being present in iron or steel. Thus, the bearing steel maycomprise at least 0.6 percent by weight of carbon, 0.007 percent byweight or less of phosphorous, and optionally other alloying element(s);the balance being iron, or iron and impurities.

Impurities may be present at levels of 0.5 percent by weight or below,preferably of 0.425 percent by weight or below. Typical impurities mayfor example be Cu, As, Sn, Sb, Pb, Ti or O, or combinations thereof. Lowlevels of impurities such as Ti and O result in a low degree of hardnon-metallic inclusions, which in combination with low levels of otherelements with tendencies to accumulate in austenite grain boundariesresult in the bearing steel having high fatigue strength. Particularly,the low amount of phosphorous according to the present invention incombination with the low degree of impurities results in a bearing steelwith high fatigue strength.

The iron may be essentially pure iron, essentially without impurities.

The amount of carbon according to the present invention lends the steela high hardness suitable for steels used for bearings. The amount ofphosphorous according to the present invention, results in a bearingsteel with high fatigue strength.

The bearing steel may have an amount of phosphorous being below 0.007percent by weight, preferably in the range of 0.003 to 0.007 percent byweight, such as 0.004 to 0.006 percent by weight. Steel with suchamounts of phosphorous may efficiently be produced at steelworks andhave high fatigue strength. Phosphorous levels below 0.003 percent byweight may be difficult and costly to produce, and levels above 0.007percent by weight does not result in the desired high fatigue strengths.

It is a benefit with the present invention that the low amounts ofphosphorous combined with the amounts of carbon result in bearing steelswith high fatigue strength.

The amount of carbon in the bearing steel may be 0.6 to 1.5 percent byweight, such as 0.7 to 1.3 percent by weight, or 0.7 to 1.1 percent byweight. Preferably the amount of carbon is 0.7 to 1.2 percent by weight,more preferably 0.8 to 1.1 percent by weight. For example the amount maybe 0.9 to 1.0 percent by weight. Such amounts of carbon lends thebearing steel suitable properties, such as high hardness. Even at suchhigh levels of carbon as for example 0.9 to 1.1 percent by weight, thebearing steels have high fatigue strengths when combined withphosphorous levels at 0.007 percent by weight or below.

Thus, the bearing steel may be regarded as being a high carbon steel orultra high carbon steel.

The bearing steel may further comprise sulphur, S, in an amount of 0.02percent by weight or below. The amount of sulphur may be 0.0001 to 0.02percent by weight, such as 0.0001 to 0.016 percent by weight, or 0.0001to 0.011 percent by weight. According to one embodiment of theinvention, the amount of sulphur is 0.0001 to 0.002 percent by weight,such as 0.0001 to 0.001 percent by weight, for example 0.0001 to 0.0002percent by weight. Such low amounts of sulphur results in the bearingsteel having high fatigue strength. According to an alternativeembodiment of the invention the amount of sulphur may be 0.002 to 0.02percent by weight, such as 0.002 to 0.013 percent by weight, for example0.003 to 0.012 percent by weight, 0.005 to 0.012 percent by weight, or0.007 to 0.011 percent by weight. Even at such high amounts of sulphur,and at such high amount of sulphur in combination with the high amountof carbon and the amount of phosphorous according to the presentinvention, the bearing steel have high fatigue strength.

The bearing steel may have a combined amount of sulphur and phosphorousof 0.02 percent by weight or less. Such a combined amount of sulphur andphosphorous may results in a bearing steel with high fatigue strength.

The bearing steel may further comprise aluminium in an amount of 0.01percent by weight or above, as alloying element. Preferably the amountof aluminium may be 0.015 percent by weight or above, or more preferably0.02 percent by weight or above. The maximum amount of aluminium may be0.05 percent by weight. Thus, the amount of aluminium in the bearingsteel may be 0.015 to 0.05 percent by weight.

The bearing steel may further comprise molybdenum, Mo, in an amount of0.1 to 0.7 percent by weight, preferably 0.3 to 0.7 percent by weight,and most preferably above 0.5 and up to 0.7 percent by weight, such as0.51 and up to 0.6 percent by weight. Such levels of Mo may result inhard bearing steels with high fatigue strength. Further such amounts ofMo may be efficient for production of bearing steels with a bainiticstructure.

The bearing steels may have a bainitic structure or be bainite hardened.It is understood that such a bearing steel also may comprise otherstructures than the bainitic structure. Preferably more than 50 percentof such a bearing steel has a bainitic structure, such as 50 to 90percent of the bearing steel.

A bainitic structure results in improved mechanical properties, withhigh toughness and high crack propagation resistance. Thus a bainiticstructure is beneficial for bearing steels and bearings due to the highloads such steels and bearings carries during normal and typical use.The combination of relatively high levels of carbon, low levels ofphosphorous, relatively high levels of Mo, and/or relatively high levelsof sulphur, according to relevant embodiments of the invention, resultsin bearing steel with high fatigue strength of the bearings.

Bainite hardening of the bearing steel may be obtained according to thefollowing method: Subjecting a steel to austenitization and quenching;subjecting the steel to an initial temperature (T1) above the initialmartensite formation temperature (Ms); lowering T1 to a temperaturebelow Ms but above the actual martensite formation temperature duringthe bainite transformation.

The method for bainite hardening results in that a bainitic structure ofthe bearing steel may be obtained efficiently with short bainitehardening times and high hardness of the bearing steel.

The hardness of the bearing steel may be above 59 HRC, for example 59 to62 HRC, or above 62 HRC.

Thus, the bearing steel or bearing may have a substantially bainiticstructure and a hardness of at least 62 HRC.

The bearing steel may also have a martensitic structure or be martensitehardened, still with high fatigue strength.

The bearing steel with an amount of Mo of 0.1 to 0.7 percent by weightmay be suitable for bearings with a material thickness from above 0 upto 150 mm. The bearing steel may have an amount o Mo of 0.1 to 0.5percent by weight, which amount of Mo may be suitable for bearings witha material thickness from above 0 up to 150 mm, such as 15 to 100 mm,for example 15 to 45 mm.

The bearing steels with an amount of Mo of 0.5 to 0.7 percent by weightmay be particularly suitable for bearings with a material thickness of45 mm or above, such as a material thickness from 45 to 150 mm, or from45 to 80 mm. Such bearings may, for example, be roller bearings with awall thickness from 45 to 80 mm.

Bearing steels used for bearings with material thickness below 15 mm mayhave an amount of Mo of below 0.35 percent by weight, or from above 0 to0.35 percent by weight. Such bearing steels may have a bainiticstructure.

Particularly, the bearing steels comprising Mo in an amount of 0.1 to0.7 percent by weight may have a bainitic structure.

The bearing steel may comprise chromium, Cr, in an amount of 1.0 to 3.0percent by weight, such 1.3 to 2.0 percent by weight. An amount of Cr of1.0 to 1.5 percent by weight may be particularly suitable for bearingswith a material thickness from above 0 and up to 45 mm, such as from 15to 45 mm. A Cr content of 1.5 to 3.0 percent by weight, such as 1.5 to2.0 percent by weight, may be particularly suitable for bearings with amaterial thickness from 45 mm and above, such as from 45 to 80 mm, or 45to 150 mm.

The bearing steels comprising Cr in an amount of 1.0 to 3.0 percent byweight may be suitable for a bainitic structure.

The optional other alloying element(s) may be selected from the groupcomprising Si, Mn, S, Cr, Ni, Mo, V, and Al, or combinations thereof.

Such optional other alloying elements may be suitable for giving thesteel suitable properties.

If present in the bearing steel, suitable levels of said optional otheralloying elements may be in the ranges of:

Silicon (Si): 0-2.5, for example 0.0001-2.5, percent by weight.

Manganese (Mn): 0-2, for example 0.0001-2, percent by weight.

Sulphur (S): 0-0.02, for example 0.0001-0.02, percent by weight.

Chromium (Cr): 0-3 percent by weight.

Nickel (Ni): 0-1, for example 0.0001-1, percent by weight.

Molybdenum (Mo): 0-1 percent by weight.

Vanadium (V): 0-1, for example 0.0001-1, percent by weight.

Aluminium (Al): 0.01-0.050 percent by weight.

Such levels of alloying elements may result in low degree of hardnon-metallic inclusions and in a bearing steel with high fatiguestrength

According to a second aspect of the invention, there is provided abearing comprising or consisting of bearing steel comprising at least0.6 percent by weight of carbon, 0.007 percent by weight or less ofphosphorous, and optionally other alloying element(s); the balance beingiron, or iron and impurities.

It may be preferred that the bearing consists of or essentially consistsof at least 0.6 percent by weight of carbon, 0.007 percent by weight orless of phosphorous, and optionally other alloying element(s).

Thus, bearings may be provided with the properties of the bearing steelas discussed above. Thus, bearings may be provided having high fatiguestrength.

The bearings may be selected from the group comprising ball bearings androller bearings; wherein the ball bearings may be selected from thegroup comprising deep groove ball bearings, angular contact ballbearings, thrust ball bearings, angular contact thrust ball bearings andself-aligning ball bearings, or combinations thereof; and wherein theroller bearings may be selected from the group comprising cylindricalroller bearings, spherical roller bearings, cylindrical roller thrustbearings, needle roller bearings, toroidal roller bearings, CARB®toroidal roller bearings, combined needle roller bearings, taperedroller bearings, tapered roller thrust bearings, needle roller thrustbearings, spherical roller thrust bearings, combined cylindricalroller/taper roller bearings, track runner bearings, and indexing rollerunits, or combinations thereof. At least one component of the bearing,such as a rolling element or a bearing ring of the bearing may comprisethe bearing steel according to the invention.

The bearings may be a combination of ball bearings and roller bearingsselected from the groups above.

The bearing may have a material thickness of above 0 and up to 150 mm,such as above 0 and up to 80 mm. Thus, the bearing may have a materialthickness of above 0 and up to 45 mm, such as 15 to 45 mm, or thebearing may have a material thickness of 45 mm or above, such as 45 to80 mm, or 45 to 150 mm.

The discussions above concerning the bearing steels related to the firstaspect of the present invention may also be relevant for the bearings ofthe second aspect of the invention concerning bearings. References tothese discussions are hereby made with regards to the bearings.

EXAMPLE

In the following comparative example benefits with the present inventionwill be appreciated and understood. It is understood that the example isincluded to improve the understanding of the present invention and thatthe example not in any way may be understood as being limitingconcerning the scope of the present invention.

Two steel heats A and B were used to cast ingots A and B. The ingots Aand B were forged to 350 mm round billets. The billets A and B were cutand machined into rotating beam samples A and B. The samples A and Bwere bainite hardened using a conventional bainite transformation cyclewith a salt bath transformation at 235° C. The samples A and B werefinished by hard turning, grinding and polishing. Chemical compositionsof the samples A and B obtained from the heats A and B are disclosed inTable 1. Samples A are comparative.

TABLE 1 Chemical compositions of bearing steels obtained from heats A(comparative) and B. p.p.m. Percent by weight of elements in steel insteel Sample C Si Mn P S Cr Ni Mo Cu V Al As Sn Ti O A 0.95 0.47 0.930.014 0.004 1.85 0.16 0.54 0.18 0.006 0.029 0.008 0.009 13 4.6 B 0.950.43 0.95 0.005 0.010 1.93 0.19 0.58 0.24 0.006 0.034 0.008 0.009 13 5.0

As illustrated by table 1, samples A and B differs in that the amount ofphosphorous is considerably lower in samples B compared to samples A;samples A have a phosphorous content being 2.8 times higher than that ofsamples B. It can also be noted from table 1 that the sulphur content insamples B is 2.5 times higher than the sulphur content of samples A.Further, the amount of Al in sample B is 17% higher than in comparativesample A. The contents of the other elements are identical or similarwhen samples A and B are compared.

Rotating beam fatigue tests and stair case tests:

Several samples A and B were tested at a constant stress level of 1060MPa and stair-cases were produced. The results from the tests areillustrated by FIGS. 1 and 2. From the tests and the results it isconcluded that the median life at the constant stress tests is at leastfive times higher for the samples B according to the invention comparedto the comparative samples A.

Stair case tests of samples A and B, as illustrated in FIG. 2, show asignificant improvement in the fatigue limit when samples B according tothe invention are compared to comparative samples A.

It is evident from the tests that the a bearing steel with the chemicalcomposition according to samples B in table 1 results in considerablyimproved properties including improved fatigue strength. For example itwas concluded that the low amounts of phosphorous had a beneficialeffect on the improved fatigue strength.

According to one embodiment of the invention, the bearing steel maycomprise 0.9 to 1.1 percent by weight of carbon, 0.004-0.007 percent byweight of phosphorous, 0.5 to 0.7 percent by weight of molybdenum, andoptionally other alloying element(s); the balance being iron, or ironand impurities. Such a bearing steel results in high fatigue strength.

According to another embodiment of the invention, the bearing steel maycomprise 0.9 to 1.1 percent by weight of carbon, 0.004-0.007 percent byweight of phosphorous, 0.5 to 0.7 percent by weight of molybdenum, 0.002to 0.016 percent by weight of S, and optionally other alloyingelement(s); the balance being iron, or iron and impurities. Such abearing steel results in high fatigue strength.

1. A bearing steel comprising at least 0.6 percent by weight of carbon,no more than 0.007 percent by weight of phosphorous, with the balancebeing other alloy elements and one of iron, or iron and impurities. 2.The bearing steel according to claim 1 wherein the amount of phosphorousis 0.007 percent by weight.
 3. The bearing steel according to claim 1,wherein the amount of carbon is 0.6 to 1.5 percent by weight.
 4. Thebearing steel according to claim 3, wherein the amount of carbon is 0.8to 1.1 percent by weight.
 5. The bearing steel according to claim 1,further comprising sulphur in an amount of no more than 0.02 percent byweight as alloying element.
 6. The bearing steel according to claim 1,further comprising aluminium in an amount of no less than 0.015 percentby weight or above, as alloying element.
 7. The bearing steel accordingto claim 1, further comprising molybdenum in an amount of 0.1 to 0.7percent by weight as alloying element.
 8. The bearing steel according toclaim 1, comprising 0.9 to 1.1 percent by weight of carbon, 0.003 to0.007 percent by weight of phosphorous, 0.5 to 0.7 percent by weight ofmolybdenum, 0.002 to 0.016 percent by weight of sulphur.
 9. The bearingsteel according to claim 1 wherein the bearing steel has a bainiticstructure.
 10. The bearing steel according to claim 1, wherein thealloying element(s) are selected from the group consisting of Si, Mn, S,Cr, Ni, Mo, V, and Al.
 11. A bearing comprising bearing steel having atleast 0.6 percent by weight of carbon, no more than 0.007 percent byweight of phosphorous, with the balance being other alloy elements andone of iron, or iron and impurities.
 12. The bearing according to claim11, wherein the bearing has a material thickness of 45 mm or below. 13.The bearing according to claim 11, wherein the bearing has a materialthickness of no less than 45 mm.
 14. The bearing steel according toclaim 1 wherein the amount of phosphorous is in the range of 0.004 to0.007 percent by weight.
 15. The bearing steel according to claim 1,further comprising sulphur in the amount of 0.002 to 0.016 percent byweight, as an alloying element.
 16. The bearing steel according to claim1, further comprising molybdenum in an amount of 0.4 to 0.7 percent byweight as an alloying element.
 17. The bearing steel according to claim1, further comprising molybdenum in an amount of no less than 0.5 and upto 0.6 percent by weight as an alloying element.
 18. The bearingaccording to claim 11, wherein the bearing has a material thickness of15 to 45 mm.
 19. The bearing according to claim 11, wherein the bearinghas a material thickness of 45 to 80 mm.